If you want to jailbreak the iPhone 3GS on iOS 4.1 beta, then here is a guide to do that with Sn0wbreeze 1.8.
But this guide is only for the iPhone 3GSs with old bootrom as Sn0wbreeze doesn’t support the new one. Follow the steps below to jailbreak iPhone 3GS on iOS 4.1 with Sn0wbreeze.

iPhone 3G users can jailbreak their devices on 4.1 beta using Redsn0w. Its guide is posted here.

NOTE : If you have already got an unlock, don’t update to 4.1.

1. Make sure that your iPhone 3GS is already jailbroken on iOS 4.

Jailbreak iPhone 3GS on iOS 4

2. Connect your iPhone 3GS with the computer.

3. Take a backup of your data in iTunes.

4. Now download the iOS 4.1 beta firmware for your device from here.

5. Also download Sn0wbreeze 1.8 beta from here.

6. Now start Sn0breeze 1.8.

7. Select ” Simple Mode “.

Jailbreak iPhone 3GS 4.1 Beta with Sn0wbreeze

8. Browse to the iOS 4.1 beta .ipsw file and choose it and Sn0wbreeze verifies it. Then move forward.

Jailbreak iPhone 3GS 4.1 Beta with Sn0wbreeze

9. In the next screen, when Sn0wbreeze asks you ” Do you want to activate your iPhone? “, click on ” Yes “. Only if you have an officially supported carrier like AT&T, click ” No “.

10. Now Sn0wbreeze starts to cook the iOS 4.1 beta firmware. Wait till that process ends.

11. When it is finished, chose your iPhone 3GS from the devices list in iTunes.

12. Holding the Shify key, click on the Restore button.

13. Now browse to the new iOS 4.1 beta firmware cooked by Sn0wbreeze and select it.

14. Wait till iTunes restore your iPhone 3GS to this jailbroken iOS 4.1 beta firmware.

Jailbreak iPhone 3GS 4.1 Beta with Sn0wbreeze

That’s it! Now you have a completely jailbroken iPhone 3GS in your hands running iOS 4.1 beta.